Drivers for the growth of high-quality, lighting devices, such as light-emitting diodes (LEDs), include demand for large screen televisions, outdoor/landscape lighting luminaries, interior illumination in the transportation sector (airplanes subways, ships, etc.), and in particular, automobiles. The demand for much higher-quality LEDs has begun to grow significantly and is expected to continue to grow as automotive manufacturers have committed to introducing models with LED forward lighting.
Since blue light-emitting diodes were first commercialized by Nichia in the mid-1990s, phosphor research and development programs at many companies have focused on re-examining their portfolios to discover materials compatible with ultraviolet (UV), violet, and blue LED wavelengths. General Electric (GE), Nichia, OSRAM (with its parent, Siemens, and with Symyx), Philips and Toyoda Gosei employ combinatorial analysis techniques to create, isolate, and test phosphor materials and morphologies.
Generally, in solid-state lighting devices, the LED is encapsulated in a suitable encapsulant. As a result, light-trapping due to Fresnel reflection and total internal reflection of photons generated within the LED at the chip-encapsulant interface currently limits the external quantum efficiencies of solid-state lighting devices to 30% of the potential efficiency. Although some of these losses are associated with absorption by the metal electrodes, phosphor losses, and losses by the reflector cup, the external quantum efficiency can be substantially enhanced by decreasing the light-trapping due to Fresnel reflection and total internal reflection, Both Fresnel reflection and total internal reflection are a result of the difference in refractive indices of the adjacent materials on each side of the chip-encapsulant interface. The greater the difference between the refractive indices of the underlying chip and the encapsulant, the larger the back reflection and the smaller the escape cone. This results in a reduced external quantum efficiency.
For example, current LEDs composed of indium-gallium-nitrogen have a refractive index of approximately 2.48. While typical encapsulants, such as silicone and/or epoxy, have a refractive index of approximately 1.5, in some cases 1.7. In the case of composite encapsulants, they typically have a refractive index equal to the volume average of their components provided that the components are significantly smaller than the wavelengths of light the encapsulant is designed to act upon. With the refractive index difference of 2.48 to 1.5 (or 1.7), there is a loss in the light extraction efficiency of the solid-state device comprising the LED and encapsulant.
Overall efficiency (in lumen/watt) and brightness (in lumen/device) of solid-state lighting devices can be significantly increased by improving the efficiency by which photons generated within the LED are extracted. One way to improve efficiency would be by improving the light extraction capability of the encapsulant.
An approach to this efficiency loss has to be use semiconductor nanocrystals in an encapsulant. Semiconductor nanocrystals may be generally comprised of spherical nanoscale crystalline II-VI, III-V, or IV-VI materials (although oblate and oblique spheroids can be grown as well as rods and other shapes) that have a diameter between 1 nanometer (nm) and 20 nm. In the strong confinement limit, the physical diameter of the nanocrystal is smaller than the bulk exciton Bohr radius causing quantum confinement effects to predominate. In strong confinement, the nanocrystal is a zero-dimensional system that has both quantized density and energy of electronic states where the actual energy and energy differences between those states are a function of both the nanocrystal composition and physical size (i.e. geometry). Larger nanocrystals have more closely spaced energy states and smaller nanocrystals have the reverse. Because interaction of light and matter is determined by the density and energy of electronic states, many of the optical and electric (optoelectric) properties of nanocrystals can be tuned or altered simply by changing the nanocrystal geometry (i.e. physical size).
Single nanocrystals or monodisperse populations of nanocrystals exhibit unique optical properties that are size tunable. Both the onset of absorption and the photoluminescent wavelength are a function of nanocrystal size and composition. The nanocrystals will absorb all wavelengths shorter than the absorption onset however, photoluminescence will always occur at the absorption onset. The bandwidth of the photoluminescent spectra is due to both homogeneous and inhomogeneous broadening mechanisms. Homogeneous mechanisms include temperature dependent Doppler broadening and broadening due to the Heisenberg uncertainty principle, while inhomogeneous broadening is due to the size distribution of the nanocrystals.
In an approach, metal oxide nanocrystals (i.e. zinc oxide, titanium oxide, etc.) have been dispersed in gelatin, polymer, silicone, epoxies, and sol-gels to form encapsulants. However, these nanocrystals have limited solubility in given matrix material and form micron-scale aggregates when their volume fraction within the matrix materials exceeds a few percent. These aggregates, in turn, strongly scatter light. As such, they would not be preferable as encapsulants for LEDs since they would strongly scatter the light emitted therefrom, making the solid-state device less efficient.
In another approach, soluble nanocrystals have been dispersed in polymer, etc., matrix material. However, these nanocrystals could be easily disrupted or destroyed in the presence of solvents and are unsuitable for many applications.
Shustack, et al., in U.S. Pat. No. 6,656,990, discloses a curable material including metal oxide nanocrystals in a matrix material. In the curable material, metal oxide nanocrystals are linked to a polymer matrix via metal-organic linking agents, where the metal atoms of the metal-organic linking agent link to the oxygen atoms of the metal oxide nanocrystals. Since metal oxides generally do not have a higher refractive index, the curable material incorporating the metal oxide nanocrystals typically can not achieve a refractive index sufficient to improve the light extraction efficiency of photons emitted by an LED in a solid-state device.
Lu, et al., in “High Refractive Index Thin Films of ZnS/Polythiourethane Nanocomposites,” J. Mater. Chem., 2003, 13, 526-530, discloses a high refractive index material including zinc sulfide (ZnS) in a matrix material. In making the high refractive index material, ZnS colloids are synthesized with ligands having hydroxyl functional groups that are linked to isocyanate function groups present on an oligomer backbone in the matrix material. There are several limitations to this approach. For example, using ligands having reactive functional groups as part of the synthesis process severely limits the types and reactive groups made available. This also prevents multiple types of functional groups being attached to the same nanocrystal or to different types of nanocrystals. Moreover, the required oligomer (i.e., polymer) backbone is incompatible with other approaches, such as nanocrystal cross linking, because the selected hydroxyl functional groups on one nanocrystal can not bond directly with the hydroxyl functional groups present on nearby nanocrystals. As such, this high refractive index material may not achieve a refractive index sufficient to improve the light extraction efficiency of photons emitted by an LED in a solid-state device.
Thus, there is a need in the art to develop a high-refractive index material to associate with a lighting device, thereby increasing the light extraction efficiency of the device.